Mutations on cardiac ion channel genes can lead to electrical rhythm disturbances known as arrhythmias. Such inherited rhythm disturbances are termed ion channelopathies and include disorders such as Brugada Syndrome (BrS) and Long QT Syndrome (LQTS). In certain instances approximately 70% of channelopathy genes are elude detection. 'MAGUK' proteins scaffold ion channels underneath cell membranes, allowing for efficient electrical signaling. Abnormal expression of a MAGUK could alter ion channel function and thereby compromise electrical excitation. The prototypical MAGUK is the synapse-associated protein 97 (SAP97). SAP97 gene is abundantly expressed in the heart, and there is evidence that SAP97 interacts with ion channels and may therefore regulate excitation. We generated a unique mouse model with the gene knock out (Sap97-KO), to investigate the role of Sap97 in excitation. Sap97-KO mice have abnormalities in ECG and cell electrophysiological properties. Furthermore, a recent clinical study identified mutations in SAP97, which were associated with BrS, and probably the result of abnormally increased expression of a key potassium channel. It is our hypothesis that SAP97 is important for the assembly of cardiac ion channel proteins, and that abnormal SAP97 expression will increase arrhythmia susceptibility. We will test our hypothesis using our mouse model, as well as patient-specific stem cell-derived cardiac myocytes (iPSC-CMs). Three aims will be addressed. 1) To study arrhythmogenic mechanisms in the myocardium of Sap97-KO mice. 2) To investigate Sap97- dependent changes in the expression of potassium, sodium and calcium channels in Sap97-KO mouse cells, and in stem cell-derived cardiac myocytes (iPSC-CMs) of patients with SAP97 mutations in BrS. 3) To study arrhythmia mechanisms associated with human SAP97 mutations in single cells, as well as in layers of iPSC- CMs. Our proposed study will provide insight into molecular mechanisms of channelopathies associated with abnormal SAP97 expression, as well as provide targets for therapeutic intervention.